1. Field of the Invention
The present invention relates to a linear power amplifier, and in particular, to a feedforward-type linear power amplifier and a method for canceling intermodulation distortion signals contained in an amplified RF signal.
2. Description of the Related Art
An RF (Radio Frequency) amplifier used for a communication system includes elements having nonlinear characteristics at high power levels. An output signal amplified by the RF amplifier may include intermodulation distortion signals in the process of amplification due to the nonlinear characteristics of the RF amplifier. Such intermodulation distortion signals cause interference and distortion at an operating frequency range of the RF amplifier.
There are several known methods for controlling a linear power amplifier (LPA) which suppresses such distortion signals: a first known method is a predistortion method for generating an inverse distortion signal to apply it to an input end of the amplifier; a second known method is a negative feedback method for negatively feeding back the distortion signals together with the its output signal to an input end to suppress the distortion signals; and a third known method is a feedforward method for extracting the distortion signals and inverting phase of the distortion signals to cancel the distortion signals. The feedforward-type linear power amplifier is widely used in a base station of a satellite or mobile communication system. Though the feedforward-type linear power amplifier has an improved efficiency in suppressing the distortion signals compared to the other types of the linear power amplifiers, it requires a large capacity and is complex in structure, thereby causing increased power loss.
FIG. 1 illustrates a block diagram of a general feedforward-type linear power amplifier, in which the amplifier extracts only intermodulation distortion signals from the output thereof and combines the intermodulation distortion signals with the output signals in opposite phase, thereby improving a C/I (Carrier to Intermodulation) ratio.
Referring to FIG. 1, a power divider 111 divides the RF input signal, and outputs the divided RF signals through a main path and a sub path. The power divider 111 may include a directional coupler. A main power amplifier 112 amplifies the divided RF signal received from the power divider 111 via the main path. A second delay line 121 receives the RF signal output from the main power amplifier 112 to delay the RF signal for an error amplification time.
A first delay line 114 delays the divided RF signal on the sub path, while the main power amplifier 112 amplifies the divided RF signal on the main path. A first variable attenuator 115 attenuates a magnitude of the RF signal on the sub path received through the first delay line 114. A first variable phase shifter 116 receives an output of the first variable attenuator 115, to control phase of the attenuated RF signal.
A power divider 113 is connected to an output end of the main power amplifier 112 to divide the output of the main power amplifier 112. The power divider 113 may include a directional coupler. A signal canceler 117 cancels the non-amplified RF signal on the sub path from the amplified RF signal output of the main power amplifier 112, so as to detect only the intermodulation signals generated during amplification of the RF signal.
A second variable attenuator 118 receives the intermodulation signals detected by the first signal canceler 117 to control a magnitude of the intermodulation signals. A second variable phase shifter 119 receives the intermodulation signals from the second variable attenuator 118 to control phase of the intermodulation signals. An error amplifier 120 amplifies the intermodulation signals from the second variable phase shifter 119. A power combiner 122 combines an output of the error amplifier 120 with an output of the second delay line 121. The power combiner 122 may include a directional coupler.
FIGS. 2A through 2E illustrate characteristics of signals generated at each part of the linear power amplifier of FIG. 1 in a case where two tones are applied thereto. Specifically, FIG. 2A shows the RF input signal, FIG. 2B the RF signal amplified by the main power amplifier 112, FIG. 2C the intermodulation signal extracted by canceling the RF input signal of FIG. 2A from the amplified RF signal of FIG. 2B, FIG. 2D the amplitude-controlled signals having an opposite phase to that of the intermodulation signals of FIG. 2C, and FIG. 2E a final output signal with the intermodulation signal being suppressed by combining the RF signal of FIG. 2B with the extracted intermodulation signal of FIG. 2D.
Referring to FIGS. 1 through 2E, operation of the feedforward-type linear power amplifier will be described hereinbelow. The RF input signal is divided by the power divider 111 and applied to a node NA of the main path and a node NA' of the sub path. Here, the divided RF signals have the same magnitude and phase, as shown in FIG. 2A. The input signal on the main path is amplified by the main power amplifier 112. The output signal of the main power amplifier 112 includes the intermodulation distortion signals with the amplified RF signals, as shown in FIG. 2B. The signal canceler 117 cancels the RF input signal of FIG. 2A from the signal of FIG. 2B, to detect the intermodulation signals as shown in FIG. 2C.
The variable attenuator 118 attenuates the intermodulation distortion signals from the signal canceler 117 so as to make the output signal levels coincide with the intermodulation distortion signals amplified by the main power amplifier 112. The variable phase shifter 119 inverts the intermodulation signals by 180.degree.. Accordingly, as shown in FIG. 2D, the intermodulation signals generated from the variable phase shifter 119 have an opposite phase to the intermodulation signals contained in the output of the main power amplifier 112. The error amplifier 120 amplifies the intermodulation signals of FIG. 2D, and the second delay line 121 delays the amplified signal of FIG. 2B on the main path, so that the delayed signal and the output of the error amplifier 120 reach the power combiner 122 concurrently. The power combiner 122 combines the intermodulation signals contained in the amplified signal of FIG. 2B from the second delay line 121 with the inverted intermodulation signals of FIG. 2D to cancel the intermodulation signals, thereby generating the final output signal as shown in FIG. 2E.
An output level of the RF amplifier for use in a base station of a mobile communication system or a personal communication system (PCS) varies, as a subscriber uses the system. With the variation of the output level, the input signal level applied to the RF amplifier is also varied, which results in a change of the characteristics of the intermodulation distortion signals generated from the RF amplifier. Accordingly, the variable attenuators 115 and 118, and the variable phase shifters 116 and 119 connected to the input and output ends of the signal canceler 117 should be continuously readjusted to optimize the linear characteristics. Furthermore, though the input level of the RF amplifier is maintained at a constant level, the output characteristic is changed according to the operational condition of the RF amplifier. Therefore, the variable attenuators 115 and 118 and the variable phase shifters 116 and 119 should be continuously readjusted to optimize the linear characteristics.
A conventional method for controlling the variable attenuators 115 and 118 and the variable phase shifters 115 and 119 is to apply a pilot tone to a pre-stage of the power divider 111 and input/output ends of the RF amplifier to detect the pilot tone at the output ends of the signal canceler 117 and the power combiner 122. However, in the case of using such a pilot tone, the RF amplifier has a decreased output power and needs various supplemental devices. Thus, the RF amplifier becomes complex in structure, and cannot control the attenuation and phase of the signal in real time.
Furthermore, in the case of using a digital signal processor, it is possible to extract, search and analyze the input signal and intermodulation distortion signals at the input end of the power divider 111, the output ends of the signal canceler 117 and the power combiner 122, and according to the analysis, control the variable attenuators 115 and 118 and the variable phase shifters 116 and 119. However, in this case, since the RF amplifier also needs various additional devices, the system becomes complex in structure and cannot control the variable attenuators 115 and 118 and the variable phase shifters 116 and 119 in real time.